OEB Seminar

Nicholas Strausfeld, The University of Arizona, will give a talk on "Arthropod Brains and Their Cambrian Antecedents: Correspondence, Divergence, and Evolutionary Stability."

Abstract: The evolutionary exuberance of euarthropods is reflected in their variety of forms, behaviors and habitats. Such profusion of shapes and functions might be expected to reflect a corresponding diversity of brains and nervous systems. However, although arthropod brains comprise a variety of discrete computational centers that mediate sensory reconstruction, learning, memory and action-selection, contemporary studies show that the overall organization of brains and ventral ganglia appears to be constrained to a unique set of motifs, referred to as ‘ground patterns.’ The discovery of fossilized brains and ventral nerve cords from China’s Chengjiang and Xiaoshiba and Canada’s Burgess Shale fauna resolve these neural ground patterns as very ancient, spanning a range of about 518–505 million years. This finding has led to the recognition that four ground patterns identified in the brains and ventral ganglia of lower Cambrian arthropods correspond to the ground patterns of four major panarthropod lineages existing today. This implies an evolutionary stability of neural arrangements over an immense time span.

Current understanding of the phylogeny and time of origin of four panarthropod lineages allows the allocation of divergent brain centers to each ground pattern and informs us of when major transformations of higher brain centers are likely to have occurred in the early Cambrian. Within a lineage, divergent arrangements of circuits within brain centers that we see today reflect evolved modifications of the ground pattern but not fundamental departures from it. Yet, we are still confronted with an enigma: if the four ground patterns we observe are evolutionarily stable over more than half a billion years, what then accounts for the enormous diversity of behaviors across panarthropods? Or is this diversity deceptive? Does each ground pattern define the degree to which a lineage can diversify within behavioral space? What is it about the pancrustacean cephalic ground pattern that endows that clade, more than chelicerates or myriapods, with a richness of behavioral repertoires that include all manner of innovations, such as those involving flight and eusociality that predate convergent achievements by vertebrates? Such questions address a future line of research: that of determining what in the evolution of the nervous system has permitted behavioral exuberance in one lineage and constraint in another.